This invention relates generally to methods and apparatus for electromagnetically vibrating articles which are subject to oscillating fatigue stresses in normal service. More particularly, it relates to improved methods and apparatus for inducing eddy currents in a non-ferrous cantilevered article so as to test the same in bending or torsional modes.
Previous techniques for the vibration-testing of cantilevered blades and the like have employed pneumatic, mechanical, and electromagnetic means for inducing vibration. For instance, vibration by pneumatic means is described in U.S. Pat. No. 3,005,334, for "Apparatus for Non-Destructive Inspection of Cantilevered Members," issued to E. R. Taylor, et al, Oct. 25, 1961. That method is, however, subject to the disadvantages of being noisy and failing to provide close control of blade vibration or temperature. A different vibration technique is disclosed in U.S. Pat. No. 3,023,610, for "A Method of and Apparatus for Carrying Out Fatigue Tests of Turbine Blades," issued to M. Prochazka, Mar. 6, 1962. In that approach, the blade is mounted to a cantilevered non-magnetic flexure arm. An armature carried by the flexure arm is vibrated by an a.c.-driven electromagnet mounted to a suitable base plate. In that method of vibration the energy is applied to the flexure arm rather than the blade itself, and thus the test conditions do not closely approach actual operating conditions in an axial-flow compressor. Also, that technique is suitable only for vibration-testing in the bending mode.
An electromagnetic system which more closely approaches actual operating conditions is disclosed in U.S. Pat. No. 3,600,934, for "Eddy-Current System for Vibration-Testing of Cantilevered Non-Ferrous Articles", issued to D. E. Hendrix and H. A. Kermicle on Aug. 24, 1971. In that system, an aluminum-alloy blade is cantilevered from a support block. The blade is vibrated in the bending mode or in the torsion mode by an electromagnet, and a permanent magnet is provided to spread the pulsating lines of force so as to increase the amount of blade vibration. A proximity probe generates an a.c. signal having the same frequency as the vibrated blade and having an amplitude proportional to the amplitude of blade vibration. The a.c. signal is shifted in phase, power-amplified, and fed to a half-wave rectifier. The resulting rectified voltage is the drive signal for the electromagnet. That system is subject to certain disadvantages, one being that the temperature of the blade under test increases appreciably. This may introduce errors in the measurement, since the fatigue properties of the blade are temperature-dependent. Heating of the blade occurs as a result of eddy currents flowing therein and of certain magnetic effects to be described hereinafter. Heating of the core of the electromagnet occurs for the same reasons, and this also is undesirable, since it may transfer heat to the blade under test.